Research Article pubs.acs.org/journal/ascecg
Simple Synthesis of Eco-Friendly Multifunctional Silk-Sericin Capped Zinc Oxide Nanorods and Their Potential for Fabrication of Hydrogen Sensors and UV Photodetectors Chuan-Chung Chuang,† Adhimoorthy Prasannan,‡ Bohr-Ran Huang,§ Po-Da Hong,*,†,‡ and Ming-Yu Chiang∥ †
Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, 43, Keelung Road, Section 4, Taipei 10607, Taiwan ‡ Department of Materials Science and Engineering, National Taiwan University of Science and Technology, 43, Keelung Road, Section 4, Taipei 10607, Taiwan § Graduate Institute of Electro-Optical Engineering and Department of Electronic and Computer Engineering, National Taiwan University of Science and Technology, 43, Keelung Road, Section 4, Taipei 10607, Taiwan ∥ Danee International Biotechology Co., Ltd., 265, Jhong Sing N. Street, Sanchong District, New Taipei City 24158, Taiwan
ABSTRACT: The present investigation describes a new eco-friendly, low cost material-based approach for sensor device fabrication. In this study, sericin capped zinc oxide nanorod (ZNR)-based H2 gas sensors and UV photodetectors were fabricated by incorporating silk-sericin with ZnO. The proposed sensors have higher sensitivity to H2 and greater photoresponsivity. The sericin capped ZNRs were fabricated from a degummed waste sericin solution using an inexpensive hydrothermal method. The sericin capped ZNRs show excellent H2 gas response (17.8%) and photoresponse with a fast response time under UV illumination due to sericin coating on the surface of the ZnO. Sericin is a water-soluble protein with strong polar groups as side chains, such as carboxyl, amino, and hydroxyl groups, which can easily interact with zinc particles through electrostatic interactions, resulting in the unique and improved ZnO sensor behavior. This interaction was characterized by various analytical techniques and compared with as-grown ZNR. The gas sensing property and UV photoresponse were evaluated for both as-grown ZNRs and sericin capped ZNRs as functions of the H2 concentration and time, respectively. Under 365 nm UV illumination, the sericin capped ZNR possesses an ultrahigh photoresponse of 408.4, which is 40 times better than that of the as-prepared ZnO devices (10.3). Moreover, the sensing response for the sericin capped ZNRs shows a complete recovery to the original level after evacuation of the H2 and UV illumination in each cycle, indicating complete desorption and decomposition of the main adsorbed moieties. The sericin capped ZNR presented in this work shows enhanced, sustained, and reversible H2 gas sensing and fast switching speed in the UV region and can be employed for the development of inorganic−organic novel materials utilizing the biomass from industrial waste. KEYWORDS: Biorenewable, Silk-sericin, ZnO photoresponse, UV photodetector, H2 sensors
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wastewater produced.1 In particular, a substantial quantity of wastewater is released during the degumming process which is used for the removal of the external sericin coating prior to the
INTRODUCTION The continuous consumption of nonrenewable resources by industrial activities has caused the exhaustion of these resources and the generation of ecological problems like pollution. Silk based textile manufacturing is among those industries where the demand for water usage is high and can have a significantly adverse impact on the environment due to large volumes of © 2017 American Chemical Society
Received: January 2, 2017 Revised: March 4, 2017 Published: March 29, 2017 4002
DOI: 10.1021/acssuschemeng.7b00012 ACS Sustainable Chem. Eng. 2017, 5, 4002−4010
Research Article
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photoconductive sensor devices is mainly dependent on the active materials and the quality of the property of the junctions. Optimization of the properties of ZnO for efficient sensor device application may be achieved through the inclusion of active materials in the ZnO. Sericin can act as a natural template, with many electron-donating groups on the surface, to which metallic ions can adhere. Controlling the formation of the crystals through electrostatic interaction15,16 may enhance the transport of electrons and could influence the sensing behavior of the ZnO. Thus, the introduction of sericin provides a new eco-friendly approach suited for the synthesis of novel ZnO nanorod (ZNR) composites for potential device fabrication. In this work, the influence of sericin deposition on the ZNR surface and the physicochemical, hydrogen gas sensing. and UV photodetecting properties were evaluated for the production of a eco-friendly, low cost, and simple materials and method. The operation and sensitivity of sericin capped ZNR-based UV photodetectors and H2 gas sensors were studied at different bias voltages. The obtained sericin capped ZNR devices show excellent hydrogen gas sensing and UV photosensing performance. Devices were fabricated by the incorporation of silk-sericin with ZnO due to the interaction between ZnO and silk sericin through the strong polar groups acting as side chains, such as the carboxyl, amino, and hydroxyl groups, which can easily interact with metallic particles resulting in unique and improved electrical properties.
dyeing process. About 25−27% of the original weight of the silk is discarded with the wastewater. Sericin is a globular protein, also called silk-gum, and it appears as a tube coating the outside of the silk fibroin, with a broad molecular weight distributed between 10,000 and 300 000 Da.2 Sericin has a unique functionality and a special molecular structure and outstanding moisture absorption properties with numerous biological activities such as antioxidation, tyrosinase activity inhibition, and anticancer activity.3,4 Hence, sericin from silk industrial waste can be utilized in a variety of fields such as the production of cosmetics, biomaterials, and textiles.5 The recovery and utilization of a valuable raw material like sericin from industrial wastewaters is desirable not only for the fabrication of biomedical materials but also for the production of novel functional nanocomposite materials. The core components of sericin peptides can be endowed with a verity of in situ active sites for the construction of the initial seeds of metal nanoparticles and can promote their growth through rearrangement.6−8 Sericin is also remarkably biocompatible and a readily available renewable resource, which could be used as an effective and low-cost substrate for novel inorganic functional materials with a variety of promising applications.9 Efforts to achieve new multifunctional semiconductor devices with material fabrication coupled with multiple physical and optical phenomena have attracted much attention. Zinc oxide (ZnO) is an especially promising semiconducting material with a wide band gap and large exciton binding energy, which is due to its tunable electronic and optoelectronic properties, has been widely utilized in optoelectronics, photocatalysis, sensors, transducers, UV detectors, light-emitting diodes, and biomedical devices.10−14 The applications of ZnO vary depending on the shape of the crystals; therefore, much effort has been made to fabricate ZnO and its unique morphology. In recent years, monitoring and determining the quantity of flammable/toxic gas under ambient conditions has become very important to ensure both individual and ecological safety. The human senses cannot generally distinguish hydrogen gas (H2) since it is colorless, odorless, and tasteless. The detection of such a hazardous gas along with quantitative information could be used to prevent a potential explosion.10 Similarly, ZnO-based UV sensor devices have significant value in a variety of applications such as in biomedical research, high temperature flame studies, space research, and environmental monitoring and safety. The efficiency of
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EXPERIMENTAL PROCEDURES
Materials and Methods. Bombyx mori cocoons were obtained from the Danee Silk International Company in Taiwan. Zinc acetate, hexamethylenetetramine (HMT), and citric acid were purchased from Sigme-Aldrich. All of the reagents were of analytical grade and were used as received without further purification. Deionized water was used throughout the experiments. Sericin Isolation. The silk-sericin solution was obtained from Bombyx mori cocoons, which were cut and boiled for 40 min in an aqueous solution of 0.04 M citric acid and then rinsed thoroughly with water to extract the glue-like sericin proteins. The sericin solution was dialyzed against water using a dialysis cassette with a molecular cutoff weight of 3500 Da for 2 days to isolate citric acid from sericin. The degumming solution was then directly freeze-dried in order to obtain the concentrated sericin protein. The molecular weight of the obtained sericin was measured using sodium dodecyl sulfate−polyacrylamide gel electrophoresis (SDS−PAGE). A polyacrylamide gel prepared by using
Scheme 1. Schematic Illustration of the Fabrication of Sericin Capped ZNR from a Silk Degummed Solution
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DOI: 10.1021/acssuschemeng.7b00012 ACS Sustainable Chem. Eng. 2017, 5, 4002−4010
Research Article
ACS Sustainable Chemistry & Engineering 12% bis(acrylamide), which was used for gel electrophoresis. After electrophoresis, proteins were stained with 0.25% Coomassie Blue, and the gels were destained in acetic acid. The extracted sericin molecular weight ranged from 7 to 300 kDa as measured by SDS−PAGE. Fabrication of Sericin Capped ZNR Based H2 and UV Sensors. The growth of vertically aligned sericin capped ZNRs (Scheme 1) was facilitated by precoating a thin ZnO seed layer on the Si substrate. ZnO crystallites were spin-coated on the substrate to promote nucleation which strongly influenced ZNR growth and morphology. The ZnO seed layer can control the alignment of the ZNRs as well as the energy and autonomous flux of the ions. The ZnO seed layer was prepared by combining a simple sol−gel process and a spin-coating technique. Prior to the spin-coating process, the Si substrate was ultrasonically cleaned in acetone. Au interdigitated electrodes were fabricated with multifingers 100-μm in length and 5-μm in width on the premodified ZnO seed layer substrates by means of photolithography and sputtering to enhance the speed of the device. Subsequently, the seeded substrates were utilized for the synthesis of sericin capped ZNRs using a simple and low temperature hydrothermal method by immersion in the premodified solution of sericin protein, zinc acetate (ZnAc, Zn (CH3COO)2·2H2O), and hexamethylenetetramine (HMT, C6H12N4) at 90 °C for 3 h. The hydrogen sensing properties of the sericin capped ZNRs were characterized in a vacuum chamber with a H2 flow of 99.99% (diluted with dry air) through a mass flow controller at room temperature. The H2 was measured in terms of ppm during the gas sensing measurement process. The two sericin capped ZNR electrodes were placed on a probe holder and connected to a computer controlled by a source measurement unit (Keithley 237 series). During the UV photodetection studies, the sericin capped ZNRs were monitored under 365 nm laser illumination at a laser intensity of 1 mW. Characterization. The surface morphology of the samples was observed using a field-emission scanning electron microscope (FESEM, JSM-6500F) with an acceleration voltage of 15 kV, and field emission gun transmission electron microscope images and EDX analysis were detected by FEI Tecnai G2 F30, operating at a voltage of 300 kV. The bonding structure of the samples was characterized by Raman spectroscopy with a Lab Raman HR800, Jobin Yvon under an excitation wavelength of 632.8 nm. Thermo Nicolet Nexus 6700 instrument were used to record the ATR-IR spectra. The crystal structure of the samples was characterized by X-ray diffraction (XRD) (D2 PHASER-X-ray powder diffraction, BRUKER) using CuKα1 radiation (λ = 1.54056 Å). The photoluminescence (PL) spectra were excited by a continuous He−Cd laser (Kimmon, 1K series) with a wavelength of 325 nm at room temperature. X-ray photoelectron spectroscopic (XPS) analyses were performed on a PHI Quantera spectrometer, and PF4 (peek fit 4) software was used to deconvolute the narrow scan XPS spectra.
Figure 1. Morphological observation of obtained sericin capped ZNRs. (a) FESEM images of the top panel, cross-sectional panel, and higher magnification views; (b) HRTEM image; and (c) EDX spectrum of a sericin capped ZNR.
The results reveal that the sericin capped ZNR was composed of sericin and zinc elements including nitrogen, carbon, and oxygen. The tentative formation mechanism of sericin capped ZNR through the hydrothermal method in an aqueous medium typically comprising zinc acetate and HMT involves zinc acetate dissociation, where Zn2+ ions can form complex water molecules as in the following:
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RESULTS AND DISCUSSION Figure 1 shows the surface morphology of the prepared sericin capped ZNRs. The ZNR surface can be clearly visualized with ZNRs of a uniform size, approximately 500 nm in diameter and an average length of about 3 μm capped by a sericin covering. The sericin is well distributed over the whole surface of the ZNR. The sericin coating was obtained with a hydrothermal process, with HMT playing a role as a pH buffer to enhance the sericin interaction with ZnO. Moreover, the sericin capped ZNR structure was formed after the introduction of sericin into the reaction medium. The coating on the ZNR surface occurred due to the shadowing effect of sputtering and agglomeration during the oxidation process. Figure 1aIII depicts top and cross-sectional images of the fabricated sericin capped ZNR structures prepared from sericin solutions with a zinc salt and HMT mixture. The several nanometer thick sericin coating on the surface of the ZNR forms capped-like shapes, as confirmed by the HRTEM images in Figure 1b which clearly show a layer of sericin surrounding the ZnO. The presence of the sericin capping layer on the ZNR was confirmed by a surface EDX spectrum analysis as shown in Figure 1c.
Zn 2 + + (CH 2)6 N4 ⇌ Zn[(CH 2)6 N4]2 +
(1)
(CH 2)6 N4 + H3O+ ⇌ [(CH 2)6 N4]H+ + H 2O (CH 2)6 N4 + 6H 2O ⇌ 6HCHO + 4NH3
(2)
Zn[(CH 2)6 N4]2 + + sericin → ZnO[sericin] + 6HCHO + 4NH3
(3)
2+
Meanwhile, the hydrolysis of Zn leans to the right for the subsequent protonation of the HMT or decomposition to ammonia groups as described in eq 2. Subsequently, with the addition of the sericin solution stirred into the above mixture we obtained sericin coated ZnO by the complete release of HMT. The intricate three-dimensional structures of sericin with various 4004
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Figure 2. Structural evaluation of sericin capped ZNR: (a) X-ray diffraction patterns; (b) ATR-IR; (c) Raman spectroscopy results; and (d) PL spectrum of sericin capped ZNR and as-grown ZNR.
metal nanoparticles.18 High stabilization of the ZNR by carboxylic (−COOH), hydroxyl (−OH), and amine (−NH2) groups can help to orient the ZNR by preventing aggregation of the particles.19 Figure 2c shows the Raman spectra of sericin capped ZNR. The corresponding ZnO peaks identified for sericin capped ZNR at 377, 412, and 437 cm−1 correspond to the A1 (transverse optical), E1 (transverse optical), and E2high (longitudinal optical) first order optical phonons, respectively. The peak at about 437 cm−1 is attributed to the ZnO wurtzite hexagonal phase, which is the same as that of the as-grown ZNR with intensity variations. Several frequency vibrational modes are also observed, which are the same as the results for regenerative silk films.20 The conformation-sensitive amide I (1600−16500 cm−1) and amide III (1150−1270 cm−1) bands are mainly useful to identify sericin on the ZNR. The existence of the band at 1610 cm−1 is attributed to amide I, and amide III is identified from the band at 1219 cm−1. The existence of the amide I and III bands at 1102 (C−C) and 936 cm−1 (C−N) clearly indicate that the ZNR capped sericin has the mainly random coil conformation associated with the α-helical conformations. Moreover, sericin is a complex of amino acids and the contribution of specific amino acid interactions with ZnO can be identified through the Raman band. The vibrations of aromatic amino acids such as Tyr, Trp, and Phe residues are recognized through the bands at 850 and 830 cm−1. Also, the band at 1005 cm−1 is attributed to the residues on the ZNR surface, and COO− stretching vibrations are clearly confirmed by the peak at 1420 cm−1, which may reveal the existence of COO− groups of aspartic and glutamic acid residues upon the formation of ZNR. A comparison of the PL spectra of the sericin capped ZNR and as-grown ZNR shown in Figure 2d can help to evaluate its crystalline quality after sericin capping. The sericin capped ZNR clearly shows two emission bands centered at about 385 nm and a clear broad band for visible light emission centered about at 570 nm. The dominant and sharp UV emission can be attributed to exciton recombination with respect to the near band emission of ZnO, and the weak green emission is caused by the existence of
conformations are closely correlated to the electrostatic interactions toward metals. In addition, sericin can act as a surfactant, and its charge distribution can interact with the growth unit of ZnO crystals as Zn[(CH2)6N4]2+. Hence, ion-pairs may form between Zn[(CH2)6N4]2+ upon the decomposition of HMT, and sericin may form a coat due to electrostatic interactions and through adsorption on the surface of the zinc particles. The as-prepared zinc cores would be sheltered by the sericin proteins at the surface of the ZnO. Comparison of the X-ray diffraction patterns of the as grown and sericin capped ZNRs helps us to understand the structural information, crystallinity, and purity of the ZNR. Figure 2a presents the XRD patterns of the sericin capped ZNR. Strong sharp ZnO diffractions are a clear indication of the high crystallinity, which can be perfectly indexed to the hexagonal wurtzite phase of ZnO and is consistent with as-grown ZNR (JCPDS card no. 36-1451) with a lattice constant of a = 3.249 Å and c = 5.205 Å. Neither ZNR nor sericin phases can be detected in the patterns, which indicates that the Zn2+ forms along with sericin and is transformed into crystalline ZnO. It can be seen that all of the diffraction peaks of the sample below 20° are apparently broad and amorphous in nature, showing the coexistence of sericin proteins on the nanocrystalline ZnO. In addition, the peak intensity diffractions corresponding to the z-axis orientation (002 and 101) are stronger rather than those of other diffractions, which is in good agreement with the results obtained from FESEM images of ZNR. The presence of sericin was further confirmed by the ATR-IR spectra of the as-grown and sericin-capped ZNR. As can be seen in Figure 2b, the ATR-IR spectrum of the sericin capped ZNR shows an obvious presence of the characteristic absorptions of proteins including amide I (1625 cm−1), amide II (1511 cm−1), and amide III (1235 cm−1).17 Also, the presence of the lower intensity peaks attributed to the carboxylate functional group (1441, 1401, and 1373 cm−1) indicates the hydrolysis of an amide linkage into its basic structural units. It is reported that these moieties can stabilize ZNR through the donation of lone pair electrons to the surface of 4005
DOI: 10.1021/acssuschemeng.7b00012 ACS Sustainable Chem. Eng. 2017, 5, 4002−4010
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Figure 3. XPS analysis of sericin capped ZNR: (a) wide scan; (b) C 1s; (c) N 1s; and (d) Zn 2p.
Figure 4. Comparison of the H2 sensing response for as-grown ZNR and sericin capped ZNR (a) as a function of various deposition times with H2 concentrations from 10 to 100 ppm; (b) selectivity performed under ammonia and acetone gas environments.
spectroscopy (XPS) was used to investigate the sericin capped ZNR, as presented in Figure 3a−d showing the wide-scan survey, C 1s, N 1s, and Zn 2p, respectively. Figure 3b shows the high resolution C 1s peak, which has been divided into four different components located at around 284.6, 286.5, 287.7, and 288.6 eV which can be ascribed to Zn−C/C-C, C−N/C-O, CO and O−CO, respectively. The N 1s deconvoluted peak in Figure 3c reveals the presence of an amine group at 399.6 eV obtained through sericin-Zn bonding, and the second small peak at 401.4 eV is assigned to the protonated amine group of the sericin. Figure 3d shows the peaks at 1021.8 and 1045.1 eV corresponding to the Zn 2p doublets 2p3/2 and 2p1/2, respectively, which is indicative of sericin bonding with Zn and is in agreement with the Zn 2p peak results. Gas Sensing Performance. The sensitivity of the sericin capped ZNRs and the plain ZNRs for the sensing of hydrogen gas was investigated with various concentrations as a function of time. Figure 4 shows typical gas response curves of the sericin
structural defects and oxygen vacancies in the ZnO crystals. Such structural defects and oxygen vacancies may arise from the sericin coating on the ZNR due to the presence of functional groups such as amine, hydroxyl, and carboxylic groups.10 The high number of oxygen vacancies is beneficial for the application of the presented ZNRs in gas sensors as they can increase the electrostatic interaction between the gas molecules and the surface of the nanorods.21 The recombination of electrons from the energy level of the sericin proteins along with defects from the ZnO energy level may be the origin of the green emission for sericin capped ZNRs. Both the UV emission and visible light emission bands of the ZNRs without sericin show a decrease in intensity when compared to that of the sericin capped ZNRs. As mentioned above, sericin capped ZNRs were developed by incorporating sericin onto the ZnO surface. In order to obtain a better understanding of the chemical composition and interaction between sericin and ZnO, X-ray photoelectron 4006
DOI: 10.1021/acssuschemeng.7b00012 ACS Sustainable Chem. Eng. 2017, 5, 4002−4010
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the removal of adsorbed oxygen, thus releasing more free electrons into the sample and causing decreased resistivity. The enhanced sensing behavior of the sericin capped ZNRs can be explained as due to the influence of the adsorption of negative oxygen species on the ZnO surface, which can lead to the formation of depletion regions, significantly affecting the resistivity of ZnO. The reduction in the resistivity of the ZnO is attributed to the possible decrease in surface depletion regions due to the sericin surface reaction to the H2 and the chemisorbed oxygen moieties on the ZnO surface while exposed to a reducing atmosphere such as H2. The oxygen vacancy is response for chemisorbed oxygen species, while sensors are exposed to H2 gas, the H2 gas may replace chemisorbed oxygen species and release the trapped electron back to the conduction band, which can be a response to the high response of gas sensor behavior. Moreover, the efficiency and selectivity of the sensing performance of the prepared sericin coated ZNRs are examined in tests performed under an ammonia and acetone gas environment, and the results are presented in Figure 4. Systematic measurements of the sericin capped ZNR gas sensors’ response to H2 were made, and the evaluation results, the I−V characteristic response as a function of the H2 concentration, are presented in Figure 5. The sericin capped ZNR based H2 sensors exhibited increased output current with respect to the amount of H2 in air, 10 ppm to 100 ppm of H2 atmosphere. The increment of the output current was higher than that of the as-grown ZNRs. For example, the measured output current for the as-grown ZNR at 30 ppm of H2 was about 25 μA, but this increased to 380 μA for the sericin capped ZNR sensor. With the change in the measurement environment from air to 10 ppm, 30 ppm, and then to 100 ppm of H2, the sericin capped ZNR showed a better response than the as-grown ZNR. Higher current responses were obtained with the sericin capped ZNR based H2 sensor following further increases in the H2 concentration. The coating of the surface of ZnO with seracin can enhance the sensing behavior of the ZNR due to the adsorption of negative oxygen species on the ZnO surface because of the
capped ZNRs and as-grown ZNRs for various H2 concentrations. The cyclic gas response is explained as a decrease in the resistance with exposure to H2 gas, and the resistance retains its initial value upon removal of the sensing gas. The gas responses are defined as [(Rgas − Rair)/Rair] × 100%, where Rgas is the resistance of the sensor upon the explosion of the hydrogen gas, and Rair is noted as the resistance in an air atmosphere. Both the sericin capped ZNR and as-grown ZNR-based sensors showed an increased response with an increasing H2 concentration. However, the sericin capped ZNR sensor exhibited a significantly higher response than that of the as-gown ZNR sensor when exposed to a higher concentration of H2. This response is attributed to the characteristic behavior of the n-type semiconductor in the sericin capped ZNRs, exhibiting remarkable sensitivity to H2. The sensitivity of the sericin capped ZNR to 100 ppm of H2 is approximately 17.8% which is higher than that of as-grown ZNR (∼6.8%). The response and recovery time of the sericin capped ZNR is
